1. Field of the Invention
This invention relates to monitoring of machinery or motors generally, and more specifically to the automated diagnosis of bearings by vibrational analysis.
2. Description of the Related Art
Rotating or oscillating motors of various sizes are ubiquitous in industrial, transportation and other applications. In many such applications it is desirable to diagnose various motor or bearing problems, as for example an imminent bearing failure, in a non-intrusive and non-destructive manner. Indeed, in many applications (for example, a ship engine) interruption of service to dissasemble a bearing for inspection would be wholly uneconomical if not disastrous.
Vibrational analysis has been used to non-intrusively diagnose motor and bearing problems without the need to interrupt operation of the motor or drive system. Various motor and bearing problems are known to relate directly to the presence of excessive vibration at frequencies related to the motor speed, typically expressed in RPM (revolutions per minute). For example, vibration at twice (2xc3x97) the motor RPM frequency often indicates mechanical bearing looseness. A vibration at a frequency of three times (3xc3x97) the motor RPM frequency often indicates misalignment of the bearings as related to the shaft. Such conventional methods of vibration analysis of machinery are discussed, for example, in Heinz P. Bloch and Fred K Geitner, Machinery Failure Analysis and Troubleshooting (Vol. 2), (Gulf Publishing, 1997) in Chapter 5, xe2x80x9cVibration Analysisxe2x80x9d. Conventional methods of vibration analysis commonly extract amplitude vs. frequency information from the vibration and are sensitive to the presence or absence of certain peak values, frequencies, or the change of a vibration waveform over time.
The conventional methods of machinery vibration analysis commonly require certain a priori information regarding the motor and/or bearings under consideration. For example, to properly analyze the vibration spectrum of a turning motor, conventional techniques require knowledge of the motor RPM (at the time the vibrational data was obtained). Other information specific to the motor/bearing system is required or greatly enhances the performance, such as the number of rolling elements per bearing. The required information is specific to the motor and/or bearing in each application, and must be provided by a user (or independent measurement). In some cases the user might actually be required to physically inspect a spare bearing to obtain the information. The precise RPM of the motor operation might vary, for example in a variable speed motor, making comparisons difficult. The necessity for such application-specific information is a great inconvenience at best, and generally discourages any attempt to develop a generally applicable, low-cost, and easily installed machine vibration analyzer.
U.S. Pat. No. 4,493,042 (1985) to Shima, discloses a bearing failure judging apparatus which diagnoses certain causes of a bearing failure from the time domain and frequency spectrum of a bearing""s vibration. The method of the patent responds to localization of amplitudes of the time-domain waveform, the periods of wave pulsations, the degree of acuteness of the waveform envelope, and the presence or absence of a specific peak value in the frequency spectrum, as well as certain changes of the waveform over time. This method extracts some information from the waveform, including the presence of broad spectrum noise, or pulsate envelopes of vibration. But it does not respond to all of the various frequency components which may be present in a motor bearing vibration, which include multiple bearing tones and harmonics, corresponding to different bearing elements. Thus, the patented method does not extract all of the relevant information inherent in the vibration signal. This approach may provide some diagnostics, such as detection of lubrication shortage, foreign matter in the lubricant, or bearing scarring. It does not provide specific information as to which bearing is faulty, whether the rolling element or the cage is at fault, whether the bearing is misaligned with the shaft, or other detailed information.
Another limitation of previous systems is that the spectral resolution is poor in higher frequency ranges, where higher harmonics of defect tones lie.
Conventional vibration analysis typically uses wired vibration sensors which are often difficult to install in existing, typically crowded industrial environments. Cable installation can be expensive and time consuming, especially if it interferes with normal operation of the equipment.
In view of the above problems, the present invention is an apparatus and method which monitors the condition of a bearing/shaft assembly by forming diagnostics based on spectral analysis of the assembly""s vibration. First, Baseline parameters are determined by a microprocessor analysis of the vibration spectra, comparing actual peaks to the peaks predicted by a model. The microprocessor estimates a rotation rate and preferably the number of rolling elements in the bearing by attempting to fit the model to the actual vibrational spectrum, by varying the assumptions. Baseline profiles are formed from one or more selected tones and preferably also harmonics thereof, to characterize the bearing/shaft assembly.
An efficient xe2x80x9cdownsamplingxe2x80x9d operation preferably provides digital filtering and resampling of the signal with a small number of operations, allowing low power, miniature circuits to be used for digital signal processing.
Preferably multiple tones and profiles are used as defect indicators, including fundamental RPM, Inner ring defect tone, outer ring defect tone, cage train defect tone, and rolling element defect tone. For each tone, a profile is calculated, summarizing the amplitudes of the tone and each harmonic, and their associated frequencies.
At a later time, bearing assembly vibration is again measured. The baseline profiles and parameters are compared against later acquired vibrational characteristics, and the comparison is used to diagnose bearing and/or shaft conditions. Deviations from the baseline tone intensities and profiles provide diagnostics which indicate the condition of the bearing. Profile xe2x80x9cexceedencesxe2x80x9d and a xe2x80x9cresiduexe2x80x9d parameter are also preferably calculated and used as diagnostic indicators of bearing condition.
The apparatus of the invention preferably includes sensors for sensing the vibration, an Analog to Digital converter for digitizing the vibrational signal, a digital signal processor which xe2x80x9cdownsamplesxe2x80x9d and calculates a frequency domain representation of the signal, and a microprocessor for analyzing the signal and forming diagnostics. These are most preferably integrated and packaged together with a portable power supply and preferably a wireless data transceiver, in a compact package. The entire device can then be easily mounted on a machine in proximity to a bearing of interest, without the necessity of a cable. Diagnostics are preferably summarized and then transmitted to a user or data station for output to a user.